<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<style>

    #box {
/*        width: 300px;
        height: 400px; */
        background: red;
        overflow: hidden;
        /* float: right; */
    }
    #box>div:nth-child(1) {
        width: 100px;
        height: 100px;
        background: green;
        float: left;
    }
    #box>div:nth-child(2) {
        width: 100px;
        height: 100px;
        background: pink;
        float: left;
    }
    #box>div:nth-child(3) {
        width: 100px;
        height: 100px;
        background: yellow;
        float: left;
    }
    
    #box>div:last-child {
        width: 100px;
        height: 100px;
        clear: both;
        background: blue;
    }
    
    span {
        background: rebeccapurple;
        height: 100px;
        float: left;
    }

</style>

<body>
    
    <div id="box">
        <div></div>
        <div></div>
        <div></div>
        <div></div>
    </div>
    <span>12fwdw</span>

</body>
</html>